Generally, as the degree of integration of a semiconductor device increases, the physical dimensions of a transistor such as a gate oxide thickness, a gate spacer width and the like abruptly decrease. Specifically, in case of low power consumption product, a transistor size reduction failing to be accompanied with a lowered supply voltage needs to sustain a minimum leakage current.
There are various kinds of leakage current such as junction leakage current, gate induced drain leakage current, Ioff (punch-thru) leakage current, and the like. Because the Ioff leakage current is the largest, a threshold voltage is raised to lower the leakage current flowing in a channel region.
However, even if a doping concentration of a threshold voltage adjustment doped region is raised to raise the threshold voltage, the high doping concentration increases an electric field between a well and a source/drain junction to generate the junction leakage current. As a result, the junction leakage current is not reduced despite the raised threshold voltage.